1. Field of the Invention
The present invention relates to an improved pressure swing adsorption process for the adsorption of nitrogen from gas mixtures with zeolite pellets.
2. Description of Prior Art
The production of oxygen from air at ambient temperatures is already performed on a large scale industrially with molecular sieve zeolites (c.f., for example, Gas Review Nippon, page 13, no. 5, 1985). Such methods exploit the preferential adsorption of nitrogen in comparison with oxygen, i.e. when the air is passed through a zeolite packing oxygen and argon are collected as the product on leaving the packing. The adsorbed nitrogen may be desorbed, for example, by evacuating the packing. In this case, the process is known as vacuum swing adsorption (VSA), in contrast with the pressure swing adsorption (PSA) process, which is also known. A continuous VSA process is characterised by the following processing stages:
a) passage of air through zeolite packing (at, for example, ambient pressure of e.g. about 1 bar) and discharge of O.sub.2 -rich gas from the outlet side; PA1 b) evacuation of the packing with a vacuum pump (for example to a vacuum of approximately 100 to 300 mbar countercurrently relative to air flow); PA1 c) filling the packing with O.sub.2 -rich gas (for example to ambient pressure of e.g. about 1 bar countercurrently relative to air flow (see, for example, FIG. 1 hereinbelow)). PA1 a packing of Li zeolite X and a packing of at least one of Sr zeolite A and Sr zeolite X, PA1 a packing of Li zeolite X and a packing of at least one of Mg zeolite A and Mg zeolite X, PA1 a packing of Li zeolite X and a packing of at least one of Ca zeolite A and Ca zeolite X, PA1 a packing of Li zeolite X and a packing of at least one of a zeolite A, which has been exchanged with calcium and magnesium ions and has a molar CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been exchanged with calcium and magnesium ions and has a molar CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95, PA1 a packing of Li zeolite X and a packing of at least one of zeolite A, which has been exchanged with calcium and strontium ions and has a molar CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar SrO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been exchanged with calcium and strontium ions and has a molar CaO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar SrO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95, PA1 a packing of Li zeolite X and a packing of at least one of a zeolite A, which has been exchanged with strontium and magnesium ions and has a molar SrO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95, and of a zeolite X, which has been exchanged with strontium and magnesium ions and has a molar SrO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95 and a molar MgO/Al.sub.2 O.sub.3 ratio of 0.05 to 0.95.
In the PSA process, stage b) is performed at approximately ambient pressure of e.g. about 1 bar with purging with a portion of the O.sub.2 -rich gas. In the so-called PVSA process (a combination of VSA and PSA), separation is performed at 1.1 to 2 bar and desorption at approximately 200 to 700 mbar (minimum pressure). The object of these processes is to achieve an elevated production rate (relative to the quantity of zeolite used) and to achieve an elevated O.sub.2 yield (ratio of the quantity of O.sub.2 in the product to the quantity of O.sub.2 in the introduced air). An elevated O.sub.2 yield results in low energy demand by the vacuum pump or air compressor.
As a consequence of the three above-stated stages, there are generally three zeolite packings, i.e. three adsorbers, which are operated cyclically. In the case of the VSA process, adsorption may also be performed with 2 adsorbers (GB-A 1 559 325).
The economic viability of such adsorption plants is influenced by capital costs, such as for example quantity of adsorbent, size of vacuum pump, and in particular by operating costs, such as the electricity consumption of the vacuum pumps. Zeolites have thus been developed with which it is possible to achieve elevated levels of nitrogen adsorption, such that the quantity of zeolite used may be kept low or even reduced. Ca zeolites A, as described in EP-A-128 545, are used for this purpose.
Further developments in this area are directed towards increasing selectivity for nitrogen over oxygen.
Elevated selectivity is achieved by using lithium zeolite X (EP-A 297 542). In comparison with Na zeolite X, a higher separation factor and higher N.sub.2 loading are achieved.
Better energy consumption is also achieved with Li zeolite X in comparison with Na zeolite X (EP-A 461 478, Example 2).
In order further to optimize adsorption processes for air separation, it has been proposed to use adsorbent packings which consist of zones having different types of zeolites.
JP 87/148 304 discloses an oxygen enrichment process in which an absorber with particular arrangements of various types of zeolites is used instead of an absorber with a single zeolite packing. At the air inlet side, the adsorber contains zeolites of the Na--X, Na--Y or Ca--X type and, on the air outlet side, of the Ca--A type.
In EP-A-374 631, a Ca zeolite A with low N.sub.2 adsorption is used in the air inlet zone, and a Ca zeolite A with elevated N.sub.2 adsorption is used in the outlet zone, wherein the CaO/Al.sub.2 O.sub.3 ratio of both zeolites is approximately equal. The different N.sub.2 loading capacities are a result of different levels of activation.
EP-A 0 546 542 describes a packing arrangement in which Li zeolite X is used in the air inlet zone and Na zeolite X in the air outlet zone.
Object
The object of the invention is to provide a more energy efficient pressure swing adsorption process for the adsorption of nitrogen from gas mixtures with less polar gas components, with which process it is also possible to achieve improved O.sub.2 yields in comparison with the prior art.